Experimental Research On The Mechanisms Of Invasion By An Alien Plant (Alternanthera Philoxeroides) In Caohai Nature Reserve

Biological invasions seriously affect the stability,structure,function,and biodiversity of ecosystems.Invasive plants can invade a variety of habitats,and increased soil nitrogen can raise the incidence of invasion by alien plants.For plants that can only use vegetative propagation in their introduced range,the survival and growth of their fragments are key factors determining their dispersal and establishment.The widespread distribution of the invasive plant alligator weed?Alternanthera philoxeroides?in Guizhou province,China,has seriously threatened biodiversity in the Caohai Nature Reserve.Eutrophication of water in Caohai is the result of application of nitrogen fertilizer in the surrounding farmlands.This study aimed to investigate the effects of this plant's invasion on the species composition and diversity of aquatic and terrestrial communities in the reserve's plateau-wetlands ecosystem.It also aimed to analyze the phenotypic plasticity of alligator weed in heterogeneous wild habitats and the effects of nitrogen addition on its growth and physiology,as well as the effect of fragment type,planting orientation,and other factors on its propagation and growth.Therefore,in this study an in situ investigation of the effects of alligator weed growth on terrestrial and aquatic seral communities was conducted.We also assessed clone growth of this invasive plant in different habitats,and growth,production,and biomass distribution of its components under different soil nitrogen treatments,and shoot characteristics and growth of different vegetative propagation fragments planted in different orientations,with or without burial in the soil.The main results were as follows:?1?In Caohai nature reserve,23 plant species belonging to 19 genera and 10 families were found in the aquatic community,and the greatest number of species belonged to Gramineae.In the terrestrial community,30 species belonging to 28 genera and 14 families were found,and the greatest number of species belonged to Compositae.The compositions of plant growth form,root growth form,life form,and water ecological type differed between aquatic and terrestrial communities.The number of families,genera,and species,and the species number of each ecological type,changed differently with the increasing importance of alligator weed in the two communities.As the importance value of alligator weed in the aquatic community increased,the Shannon-Wiener diversity index increased significantly,the evenness index increased at first and then decreased,and the Simpson's index decreased at first and then increased.Though the species composition and diversity showed different trends with the increasing importance of alligator weed during the early stages of invasion in the two communities,when the invasion became serious the diversity of both communities was endangered.?2?Soil characteristics were different in four different habitats.The growth characteristics of alligator weed were significantly different among these habitats as well.The internode length was shorter and the leaf biomass ratio was larger in a sunny environment than in a shaded one.The root length was shorter and the stem biomass ratio was larger in a disturbed environment than in an undisturbed one.Many coefficients of variation of measured variables were larger within,than among,populations.There were allometric relations of power or linear functional types between components,which also differed among habitats.?3?The growth of alligator weed increased significantly with the addition of nitrogen in September and November.The stem biomass ratio increased with the addition of nitrogen in September.The root biomass ratio and root to shoot ratio decreased with the addition of nitrogen in September.Nitrogen content did not have significant effects on content of chlorophyll a,chlorophyll b,carotenoids,total chlorophyll,soluble sugar,and total sugar.Soluble protein in the leaves was the greatest at a nitrogen level of 16 g N m^-2,and was significantly different from that in other treatments.Nitrogen content did not have significant effects on peroxidase activity and malondialdehyde content.The superoxide dismutase and catalase activity increased at first,and then significantly decreased after nitrogen addition.The total nitrogen content in the stem decreased with the addition of nitrogen.The total phosphorus content increased with the addition of nitrogen.Therefore,a moderate increase in the nitrogen content of soil may promote the expansion of the alien invasive plant alligator weed.Excessive nitrogen inhibits the antioxidant activity of plants and affects their growth.?4?The node and internode adjacent mode,fragment type,and burial orientation of fragments had significant effects on the sprouting and growth of alligator weed.Fragments made from the basal node had decreased sprouting rate and delayed sprouting time.Those from the apical node had delayed rooting time and decreased root length and root biomass.The ramet of one-node fragments grew better than that of two-node fragments.Downward orientation decreased root growth.There were interactive effects of the three factors on the growth of ramets.For one-node fragments,having the basal node in an upward orientation increased the growth of the ramet.For two-node fragments,apical and basal nodes increased the shoot growth and root growth respectively,in the horizontal and upward orientations.These treatments would promote invasion by alligator weed.?5?Downward orientation decreased the growth of ramets in the fragments from the basal node above the soil surface in the one-node fragments and ramet in basal node on soil surface in two-node fragments.Upward orientation increased the growth of the ramets of the apical node above the soil surface in the two-node fragments.Treatments under the soil surface decreased the growth of the ramets.One-node fragments were affected by burying under the soil surface more than two-node fragments were.This suggests that we need to avoid residuals and scattering of fragments when using physical methods to clear away alligator weed.Sweeping away alligator weed thoroughly may efficiently prevent invasion.This study comprehensively examined the influence of alligator weed on the composition and diversity of plant communities,the strong plasticity of alligator weed,and the diversity of the external environment in which it can grow.We further enriched the field of population reproductive ecology,proposed a practical and feasible scientific method for eliminating residual plants fragments,and provided a theoretical basis for preventing the invasion of alligator weed for the protection of plant communities and the sustainable utilization of their resources.